1. Field of the Invention
The present invention relates to a semiconductor device utilizing a crystalline thin film transistor and a method of manufacturing the same and, more particularly, to a thin film transistor (TFT). The present invention can be applied to thin film transistors of any type such as planar and reverse staggered types.
2. Description of Related Art
Recently, techniques for fabricating thin film transistors on an inexpensive glass substrate have been rapidly advancing. Especially, thin film transistors utilizing a thin film semiconductor as an active layer have made remarkable progress. Silicon films are commonly used as thin film semiconductors, and thin film transistors utilizing amorphous silicon films have already been put in use.
However, progress in the development of active matrix type liquid crystal displays and the like wherein a pixel region and a driving circuit are integrated has resulted in an increased need for thin film transistors for such driving circuits operating at a high speed. As a result, conventional thin film transistors utilizing amorphous silicon films have become insufficient to achieve required characteristics.
Under such circumstances, there is a recent trend toward thin film transistors whose active layer is constituted by a crystalline silicon film obtained by crystallizing an amorphous silicon film. In a crystalline silicon film (excluding single crystal silicon films), however, the presence of trap levels of a high density at grain boundaries of the crystalline silicon results in migration of carriers through such traps which causes an off-current to flow.
Such an off-current is a current flowing through the channel region of a thin film transistor in the off-state. The off-current can prevent charge stored in the thin film transistor from being maintained for a long time when the transistor is used as a switching device for a capacitor formed by a pixel electrode or the like in a liquid crystal display or the like.